1. Field of the Invention
This invention relates to a thin film transistor used in liquid crystal display panels, image sensors or the like and its production method.
2. Description of the Related Art
Recently, the technology of forming thin film transistors or thin film diodes on a glass substrate has been realized for making large area liquid crystal display panels having a back light, contact-type image sensors and the like and under such a circumstance, research and developmental activities have been positively progressed. Particularly, the thin film transistor using a polysilicon film as a channel layer has been attracting strong attention lately as one of the most hopeful devices capable of making a thin film device having peripheral driving circuits integrated therewith.
The thin film transistor using a polysilicon film as a channel layer is generally disadvantageous in that it is larger in leak current (OFF current) than a MOSFET or thin film transistor using an amorphous silicon film as a channel layer. Such disadvantage is true when it is used as the switching device of a liquid crystal display panel and when a liquid crystal display panel driving circuit is designed. In addition, the thin film transistor is applied in many cases for devices which require a high driving voltage as the liquid crystal display panel, electroluminescent (EL) display panel or the like and as a result, such a thin film transistor that is high in withstand voltage and small in leak current is needed.
Single-crystal silicon almost does not arise a problem because of the fact that it is small in leak current. On the other hand, polysilicon has many grain boundary traps in its band gap and as a result, an electric current is easy to be flowed between the bands through these grain boundary traps, thus becoming larger in leak current than the single-crystal silicon.
The magnitude of the leak current of a thin film transistor largely depends on the magnitude of an electric field applied to a depletion layer formed at one end of the drain region, so that it has been found that the leak current can be reduced by weakening the electric field applied to the drain region end. As a result, various types of structure has been proposed previously in order to weaken the electric field at one end of the drain region. One example of them is of the offset gate structure in which a gate electrode is formed slightly apart from the drain region.
A conventional stagger-type thin film transistor of the offset gate structure is shown in FIG. 1 as an example. As shown in FIG. 1, a source region 2a and a drain region 2b made of a p or n impurity-doped polysilicon are formed on a substrate 1 at an interval. A polysilicon film 4 becoming a channel layer is formed on the substrate 1 so as to cover the source and drain regions 2a and 2b. The polysilicon film 4 is contacted to the surface of the substrate 1 between the source and drain regions 2a and 2b. On the polysilicon film 4 is formed a gate insulating film 5. Then, a first gate electrode 6 is formed thereon so as to be disposed just above the substrate 1 between the regions 2a and 2b. The first gate electrode 6 is formed so as to have its both ends spaced respectively from the one ends of the source and drain regions 2a and 2b thereby forming offset regions A between the both ends of the first gate electrode 6 and the respective one ends of the source and drain regions 2a and 2b.
Besides, an inter-layer insulating film 3 is formed on the gate insulating film 5 so as to cover this first gate electrode 6. On the inter-layer insulating film 3 is formed a second gate electrode 7 so as to be disposed just above the first gate electrode 6. The both ends of the second gate electrode 7 is slightly extended outwardly of the offset regions A thereby being overlapped with the source and drain regions 2a and 2b, respectively.
The thin film transistor as shown above makes possible that by applying a voltage to the second gate electrode 7 thereby weakly inverting the channel layer, namely, the polysilicon film 4, of the offset region A on the side of the drain region 2b, the leak current can be suppressed without reducing the ON current characteristic. With this structure, however, it is disadvantageous in that the second gate electrode 7 is required to be provided additionally and as a result, it becomes complex in structure as well as is required to form one layer additionally for wiring the second gate electrode 7. In addition, if the width of the offset region A, that is, the distance of each end of the first gate electrode 6 and the corresponding end of each of the source and drain regions 2a and 2b is increased, the maximum ON current is disadvantageously decreased thereby degrading the characteristics of the transistor itself. This means that the offset gate one shown in FIG. 1 is difficult to be realizably used in liquid crystal display panel.
Another example of the conventional stagger-type thin film transistor is shown in FIG. 2. The thin film transistor shown in FIG. 2 can be eliminated the second gate electrode 7 shown in FIG. 1 by providing the source/drain electrodes with the functions of the second gate electrode 7, resulting in the elimination of the disadvantages due to the second gate electrode 7.
A source region 12a and a drain region 12b made of impurity-doped polysilicon are formed from each other on a glass substrate 11 at an interval. A polysilicon film 14 becoming a channel layer is formed thereon so as to cover the source and drain regions 12a and 12b. The polysilicon film 14 is contacted to the surface of the substrate 11 between the source and drain regions 12a and 12b. A gate insulating film 15 is formed on the polysilicon film 14. On the gate insulating film 15, a gate electrode 16 is formed so as to be disposed between the source and drain regions 12a and 12b. The gate electrode 16 is formed so as to have its both ends spaced respectively from the one ends of the source and drain regions 12a and 12b thereby forming offset regions B between the both ends of the gate electrode 16 and the respective one ends of the source and drain regions 12a and 12b. Besides, an inter-layer insulating film 13 is formed on the gate insulating film 15 so as to cover the gate electrode 16.
This transistor is similar in structure to that shown in FIG. 1. In this transistor, however, a source electrode 18a and a drain electrode 18b connected via contact holes 19 formed passing through the inter-layer insulating film 13, gate insulating film 15 and polysilicon film 14 respectively to the source region 12a and drain region 12b are formed so as to be extended inwardly exceeding the respective offset regions B (that is, horizontally extended to the side of the gate electrode 16) on the inter-layer insulating film 13, thus having their horizontally extended ends overlapped with the gate electrode 16.
In the thin film transistor shown in FIG. 2, a voltage is not applied to the drain electrode 18b when the transistor is OFF, so that the leak current becomes small. When the transistor is ON, a voltage applied to the drain electrode 18b acts directly to the polysilicon film 14 of the offset region B, so that the polysilicon film 14 can be weakly inverted. Thus, the drain electrode 18b can provide the same functions as those of the second electrode 7 shown in FIG. 1 and as a result, the leak current can be suppressed without degrading the ON current characteristic of the transistor.
According to the structure as shown in FIG. 2, the difficult points pointed out in the preceding example can be eliminated, being advantageous in that the same effects as of the structure shown in FIG. 1 can be obtained by producing as the conventional thin film transistor. With the structure as shown in FIG. 2, such a disadvantage that the maximum ON current is decreased if the width of the offset region B is increased is still remained to be overcome.
Various types of structure including a planar, stagger and the like are known for the thin film transistor. Particularly referring to the planar-type one, it is disadvantageous that the leak current is rapidly increased when a voltage applied between the source and drain regions is increased. This is because a high electric field is applied at the end of the drain region by the voltage applied between the source and drain regions and as a result, there generates an electric field emission current between the bands. Accordingly, the planar-type one is not appropriate to be used for the application that requires a low leak current.
In the conventional thin film transistor shown in FIG. 1, however, there exist two layers consisting of the gate insulating film 5 and inter-layer insulating film 3 between the polysilicon film 4 and second electrode 7. As a result, the polysilicon film 4 in the offset region A is applied through the two layers with an electric field. Similarly, in the conventional one as shown in FIG. 2, there exist two layers consisting of the gate insulating film 15 and interlayer insulating film 13 between the polysilicon film 14 and the area of the drain electrode 18b serving to act the functions of the second electrode 7 and as a result, the polysilicon film 14 of the offset region B is applied through the two layers with an electric field. Accordingly, the voltage applied to the second electrode 7 or the drain electrode 18b is not effectively acted on the polysilicon film 4 or 14 respectively of the offset region A or B, so that there arises a problem that maximum ON current degrades when the width of the offset region A or B is made large.
In the transistor shown in FIG. 2, in addition, the voltage applied to the polysilicon film 14 of the offset region B is always equal to a drain voltage, so that the leak current suppression effect depends on the drain voltage and as a result, there arises a problem that when the drain voltage is low, satisfiable effects cannot be obtained.
Thus, an object of this invention is to provide a thin film transistor which can be made smaller in leak current than would be obtained conventionally without degradation of maximum ON current and a production method of the same.